Downhole firing apparatus is employed to form perforations in well casings and other earth formations so as to provide a flow path for hydrocarbons from the formation into the well casing where it can then be pumped to the surface. Explosive charges are often utilized in perforation guns to form holes in the casings, which can have sidewall thicknesses up to 1/2 inch. It can thus be appreciated that a substantial amount of energy must be developed as a result of the explosive charge to perforate such a casing sidewall.
When the perforating gun functions properly, the explosive charges therein go "high order", whereby weakened sidewall areas in the perforating gun are blown out by the internal explosive concussion, as are areas of the casing adjacent the perforating gun. However, in certain situations, when the explosive charges within the perforating gun do not go high order, the combustion products do not have an escape path, and thus the pressures within the perforating gun can reach 8,000 psi. A serious problem then arises as to the manner in which the internal gas pressure can be reduced so that the tubing string and perforating gun apparatus can be disassembled at the surface and repaired or replaced. It can be appreciated that with such a pressurized perforating gun, personnel at the surface of the well site are endangered in attempting to disassemble the perforating apparatus.
Another problem is known to exist in which a downhole perforating gun does function properly to perforate the well casing, but thereafter the holes in the gun become plugged, often due to high density drilling mud, or other material. The problem is especially exacerbated in very deep wells where the downhole pressures can approach or exceed 10-15,000 psi. When this occurs, the ambient pressure within the fired perforating gun is at the same pressure as the surrounding bore, and is captured therein by the drill mud. Hence, when the drill string and attached perforating gun are raised to the surface of the earth, the plugged perforating gun is highly pressurized and the situation is much like that of a misfire when the internal explosives do not go high order.
Attempts have been made to address those problems by manually piercing perforating guns at one of the thinned sidewall areas. While such an approach is effective in reducing the internal pressures, the sudden discharge of the pressure is uncontrolled and any internal debris or apparatus is shot from the gun, much like shrapnel. Hence, safety considerations to the personnel still remain a serious problem.
From the foregoing, it can be seen a need exists for apparatus and a technique for depressurizing or bleeding the internal pressures of a perforating gun in a controlled manner so as to reduce the pressure without serious harm to personnel.